1. Field of the Invention
This invention relates to apparatus for controlling the fluid level in, between and when discharging liquid from flotation cells, or tanks, which are used for separation of liquid and solid phases of an influent fluid. Specifically, this invention relates to fluid control apparatus which operates on a reduced scale, and with a specially designed valve to provide more controlled flow of fluid.
2. Statement of the Art
Flotation cells or tanks are widely used in a variety of industries, such as oil, wastewater treatment, pulp and paper, and mining and mineral reclamation, to separate an influent liquid or feed slurry into a clarified liquid phase and a solid or particulate matter phase. Flotation tanks generally operate by facilitating the flotation of solid or particulate matter, such as mineral-bearing particles, to the top of the fluid bed contained in the tank, while a liquid phase develops toward the bottom of the tank. The liquid phase may typically contain varying amounts of solids or particulates which are not completely separated from the liquid. Thus, the liquid phase may range from a relatively clarified liquid to a pulp or slurry. Flotation of the solid phase or mineral particles to the top of the tank is often facilitated by such means as introducing air into the influent liquid to form a froth which captures or binds the solid or mineral particles matter and floats them to the top of the liquid volume in the tank. The solids or concentrated mineral particles matter which have been floated to the top of the liquid level are typically removed from the tank by causing the floating material to overflow into a launder, usually positioned about the periphery of the tank.
It is important to the efficient operation of flotation cells that the liquid level in the tank be maintained within a certain specified range so that the floating mineral concentrate of froth bed also remains at a specified level in the flotation cell to optimize recovery of the solid or particulate matter. In other words, if the liquid level in the flotation cell is too low, the separated solids or mineral concentrate, also referred to herein as xe2x80x9cthe float,xe2x80x9d will remain afloat on the liquid volume and will not overflow into the launder, thereby increasing the residency time of the float. The longer the float stays in the tank, the greater the possibility that the solid or mineral concentrate will sink back into the liquid volume and decrease the efficiency of the separation process. Conversely, if the liquid level in the tank is too high, the float may move efficiently to the overflow launder, but an increased amount of liquid will overflow and enter the launder as well. An inordinate amount of liquid in the overflow launder reduces the efficiency of the later processing of the mineral concentrate.
Thus, it has been recognized for some time that it is beneficial to the operation of flotation tanks to provide means for controlling the liquid (i.e., pulp or slurry) level in the tank. Control devices which are conventionally used in industry comprise a separate tank, often termed a xe2x80x9cbox,xe2x80x9d which is positioned externally to the flotation cell. The control box is in fluid communication with the flotation cell via one or more conduits interconnected between the flotation cell and the control box. In large plant operations, control boxes are typically interconnected between two adjacent flotation cells and are in fluid communication with both flotation cells. Conventional control boxes generally include a valve positioned internally to the box which operates to let fluid flow through the box from one flotation cell to the next adjacent flotation cell, thereby modifying the liquid level in both flotation cells.
Conventional fluid level control boxes tend to be substantially the same height as the flotation cell since the liquid level in the control box is maintained at approximately the same depth as the liquid in the flotation cell. Thus, for example, the control box may range in height from five feet to twenty feet. The length of conventional control boxes may generally be just short of the diameter of the flotation cell (e.g., three to six feet or greater) and may be one to five feet wide. In many large industrial applications, several flotation cells are positioned adjacent each other and are all placed in fluid communication with one or more adjacent flotation cells so that the liquid flow from one flotation cell is directed to the next adjacent cell, and so on. Conventional liquid level control boxes are positioned between adjacent flotation cells so that the liquid flowing from a first flotation cell enters into the control box. Liquid then enters into the next adjacent flotation cell through a conduit interconnected between the control box and the second flotation cell. By so arranging the flotation cells and liquid control boxes therebetween, the liquid level in each individual cell of a grouping of flotation cells can be optimally controlled.
One of the major drawbacks encountered with use of conventional liquid control boxes is their size, which not only increases capital costs in operation of the flotation cells, but limits the area capacity and, therefore, the number of flotation cells which may be installed at a given plant site. That is, conventional fluid control boxes are so large, and must necessarily be located between adjacent flotation cells, that they take up vital space which may be used for the installation of more flotation cells or which may be used for other purposes. Furthermore, when maintenance is required on conventional control boxes, the flotation cells to which the control box is attached must be taken off line while repairs are effected.
Additionally, the control valves of known liquid control boxes are, by virtue of their configuration, unable to provide finely controlled release of liquid through the control box. More specifically, known control valves provide an initial rapid flow rate of liquid which levels off quickly as the valve is opened. The fluid flow dynamics of conventional fluid level control boxes are, therefore, less subject to finite control.
It would be advantageous, therefore, to provide a fluid level control apparatus which provides finely controlled fluid flow therethrough, which provides ease of maintenance and repair, and which reduces capital costs by reducing the size and operation of the liquid level control apparatus and by enabling more flotation cells to be installed at a plant site,
In accordance with the present invention, a flotation cell liquid level control apparatus is configured for increased control of fluid flow therethrough and is structured to be reduced in size to increase operation efficiency and to increase area capacity for the placement and operation of flotation cells. The flotation cell liquid control apparatus of the present invention may be employed in connection with various types of flotation cells, and may be employed for use in connection with one or more flotation cells to control the liquid level in one flotation cell or adjacent flotation cells.
The flotation cell liquid level control apparatus of the present invention generally comprises a vessel which is located externally to a flotation cell and is of significantly smaller area in cross section than the flotation cell, or cells, to which it is connected. The smaller size of the control apparatus provides greater efficiency in operation as compared with the large control boxes known in the prior art, and the control apparatus reduces capital costs by providing more area capacity for the placement of flotation cells. The vessel has a bottom and sides, and has first and second interior chambers formed by a divider positioned within the vessel. The divider provides a valve seat against which a movable valve body is positionable to prevent fluid from flowing between the two interior chambers. The valve and valve seat are selectively operable to control movement of liquid received from a flotation cell through the vessel.
The vessel is formed with an inlet which opens into the first interior chamber of the vessel. The inlet is positioned to receive liquid (e.g., pulp or slurry) from a flotation cell positioned upstream from the vessel. The vessel also includes an outlet which opens from the second interior chamber to provide a point of discharge of fluid flowing through the vessel. The outlet may typically be connected to another flotation cell which is positioned adjacent to and downstream from the other flotation cell to which the inlet of the vessel is attached. Alternatively, the outlet may be positioned to direct fluid to a discharge outlet for further processing. In one embodiment of the invention, both the inlet and the outlet of the vessel may each have associated therewith an isolation valve which may be operated to prevent fluid flow out of or into the flotation cell at the point of connection of the vessel to each flotation cell. Thus, the isolation valve of the inlet and outlet may be engaged so that the vessel can be taken off-line for maintenance or repair. In such an embodiment, two control apparatus may be interconnected between adjacent flotation cells so that fluid flow from a first flotation cell may be directed through two separate vessels into the adjacent and downstream cell, and at any given time, one of those vessels may be taken off-line, leaving the other vessel to provide necessary fluid control. Little or no interruption in separation processing is experienced.
The vessel of the control apparatus is structured with a divider which demarcates the interior of the vessel into a first interior chamber and a second interior chamber. The divider is formed with at least one aperture providing a passageway for fluid flowing from the first interior chamber to the second interior chamber. The aperture defines a valve seat against which a movable valve body is selectively positionable. The valve body may generally be frustoconical in shape, but the exterior surface of the valve body is especially adapted with a defined curvature, extending from the top of the frustoconical member to the bottom thereof, which insures a selectively controllable flow rate of liquid through the valve.
More specifically, prior art valves are known which use a frustoconical valve body with an exterior surface which is linear from the top of the valve body to the bottom of the valve body. A space or gap forms between the valve seat and the external surface of the valve body as the valve body moves away from the valve seat. The amount of fluid which passes through the space or gap between the valve seat and valve body can be progressively plotted as the gap widens. Experimental values show that with conventional valve bodies having a linear outer surface, the amount of fluid which moves through the valve increases very rapidly as the valve body begins to disengage from the valve seat, but also levels off rapidly to a constant flow rate, resulting in a non-linear flow rate. In the present invention, the outer surface of the valve body is curved in such a manner that the gap which forms between the valve seat and the outer surface of the valve body widens selectively to produce a fluid flow rate proportionate to the gap provided between the valve seat and valve body. In other words, a plotting of the flow rate through the valve of the present invention reveals a substantially linear increase in flow rate. As a result, the amount of fluid released through the valve can be very finely controlled.
The valve body is connected to an actuation mechanism which moves the valve body relative to the valve seat for opening and closing the valve. The actuation mechanism may be any suitable device which causes the valve body to move relative to the valve seat to selectively open or close the valve responsive to a stimulus or to an indication that the fluid level in the associated flotation cell needs to be modified (i.e, either increased or decreased). One example of a suitable actuation mechanism is an electromechanical mechanism interconnected between a movable rod attached to the valve body and a fluid level detector attached to the flotation cell which signals the movable rod to open or close the valve responsive to the fluid level in the flotation cell. Alternatively, an actuation mechanism may operate responsive to a change in pressure differential, usually detected between the pressure in the first interior chamber of the vessel and a pressure external to the vessel (e.g., the pressure in the flotation cell). Such actuation mechanism may comprise the use of one or more flexible diaphragms.
The reduced size of the flotation cell fluid level control apparatus of the present invention, as compared with conventional control devices, reduces the capital costs of operating a single flotation cell or a plurality of flotation cells because the control apparatus of the present invention is easier to build and maintain, and reduces the number of wear components required in the construction. More importantly, the reduced size of the control apparatus allows multiple flotation cells to be positioned closer together in a manifold arrangement which translates to greater area capacity for flotation cell operation and separation processes. These and other benefits of the present invention will become apparent in the description of the illustrated embodiments, which follows.